Photochromism of the natural dye 7,4'-dihydroxy-5-methoxyflavylium (dracoflavylium) in the presence of (2-hydroxypropyl)-β-cyclodextrin.

A photochromic system based on dracoflavylium, a natural pigment extracted from Dragon's blood, a resin appearing in the injury parts of the tree Dracaena draco, is studied in water. The photochromism arises from the irradiation of the trans-chalcone, giving a mixture of flavylium cation/quinoidal base as a photoproduct via cis-chalcone and hemiketal. The performance of the photochromic system can be improved in the presence of (2-hydroxypropyl)-β-cyclodextrin. A mathematical model to account for the details of the kinetics and thermodynamics of the system was deduced. The model is general for all the host-guest systems involving the flavylium network of chemical reactions with 1 : 1 stoichiometric association.